JPH079761A - Recording and erasing method and apparatus for reversible thermal recording medium - Google Patents

Abstract

PURPOSE:To realize the formation and erasure of a recording image within a short time, in a recording method utilizing the color forming reaction between an electron donating color forming compd. and an electron acceptive compd., by providing a process heating a recording medium to temp. higher than room temp. before a color forming recording process or a recording erasing process. CONSTITUTION:In a reversible thermal recording medium having a recording layer 2 containing an electron donating color forming compd. (color former) and an electron acceptive compd. (coupler), in an image forming process, the reversible thermal recording medium is heated to temp. equal to or higher than T1 by a thermal head to heat and melt the coupler and the color former to form a developed color compsn. In an image erasing process, the reversible thermal recording medium is heated to the temp. between T0-T1 by a heating roller 5 and the coupler molecule is separated from the developed color compsn. to be crystallized to erase an image. In this case, a process for preheating the recording medium immediately before heat is applied to the recording medium by the thermal head 4 is provided and, by this process, the recording medium is heated to predetermined temp. within a short time to obtain a recording image of high resolving power.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording method and a recording erasing method for a recording medium utilizing a color forming reaction between an electron-donating color-forming compound and an electron-accepting compound, and an apparatus used therefor.

[0002]

2. Description of the Related Art Conventionally, heat-sensitive recording utilizing a color-forming reaction between an electron-donating color-forming compound (hereinafter also referred to as a color-developing agent) and an electron-accepting compound (hereinafter also referred to as a color-developing agent). The medium is widely known, and the output of electronic calculators, facsimiles, vending machines, printers of scientific measuring machines, C
Widely used in RT medical measurement printers. However, all the conventional products have irreversible color development, and it is not possible to repeat the color development and the color erasing alternately.

On the other hand, according to the patent publication, there has been proposed a reversible color developing and decoloring method. For example, a combination of gallic acid and phloroglucinol is used as a color developing agent. No. 193691, Japanese Patent Application Laid-Open No. 61-237684, which uses a compound such as phenolphthalein or thymolphthalein as a developer, and a recording layer containing a homogeneous compatible solution of a color former, a developer and a carboxylic acid ester. JP-A-62-138556, JP-A-62-138
568 and JP-A-62-140881, JP-A-2-188294 in which a salt of gallic acid and a higher aliphatic amine is used as a color developer, and bis (hydroxyphenyl) acetic acid or butyric acid and a higher level as color developers. JP-A-2-188293, which uses a salt with an aliphatic amine, is disclosed. However, various problems remain in the conventional reversible thermosensitive recording medium described above, and it is not yet satisfactory.

The applicant of the present invention previously used a compound such as an organic phosphoric acid having a long-chain aliphatic group, a carboxyl compound, a phenol compound, or a hydroxyphosphonic acid as a color developer, and used it as a fluoran compound or the like as a color developing agent. By combining them, the coloring and decoloring can be easily performed only by heating, and the coloring and decoloring states can be maintained at room temperature, and the decoloring temperature is lower than the coloring temperature. In addition, a reversible thermochromic composition capable of repeating image formation and erasing many times by temperature change, and a reversible thermosensitive recording medium containing the same in a recording layer have been proposed (Japanese Patent Application No. 3-355078). . This reversible thermosensitive recording medium does not cause problems such as a decrease in color density and defective erasing even when it is repeatedly used a number of times, and can be used a number of times that is unthinkable from the prior art.

The reversible thermosensitive recording medium proposed by the present applicant is capable of repeatedly forming and erasing a record only by heating it to a predetermined temperature. In this case, any heating means can be used as long as it can be heated to the predetermined temperature. . Therefore, a thermal head, a hot stamp or the like is used for ordinary recording formation, and a heating roller, a heating lamp or the like is used for erasing the recording.
Of these, the thermal head is the most general one as in the case of using the conventional irreversible heat-sensitive recording material, and can also be used for recording / erasing. However, when the recording medium is heated from room temperature to the color recording temperature or the record erasing temperature all at once with the thermal head, the surface temperature of the thermal head is significantly higher than the temperature required for color recording or recording erasing. The surface temperature of the heated recording medium rises more than necessary, so that the life of the recording medium is unavoidably shortened. It is considered that this decrease in the life of the recording medium occurs because the recording medium is heated to an unnecessarily high temperature. On the other hand, when recording is repeated for a long time under low temperature heating using a heating roller or hot plate, the recording medium deteriorates slightly, but the heat is isotropic because the heating time is long. And the resolution of the image deteriorates, and a clear image cannot be obtained.

[0006]

DISCLOSURE OF THE INVENTION The present invention is a method of heating a reversible thermosensitive recording medium for recording and erasing a record, without heating the recording medium to an unnecessarily high temperature and for a short time. It is an object of the present invention to provide a method for forming and erasing a recorded image by heating, and an apparatus used for the method.

[0007]

The inventors of the present invention have completed the present invention as a result of intensive studies to solve the above problems. That is, according to the present invention, a recording layer containing a color former and a developer is provided, a color recording state is formed by heating and melting, and when the temperature is lower than the color recording temperature, an erased state in which recording disappears is formed. In a method including a color recording step of recording a color on a reversible thermosensitive recording medium and / or an erasing step of erasing the color recording, the color recording step and / or
Alternatively, there is provided a recording method and a recording / erasing method for a reversible thermosensitive recording medium, which is provided with a preliminary heating step for heating the recording medium to a temperature higher than room temperature immediately before the recording / erasing step. Further, according to the present invention, a recording layer containing a color former and a developer is provided, a color recording state is formed by heating and melting, and when the temperature is lower than the color recording temperature, an erased state in which recording disappears is formed. In a device including a color forming recording device for performing color recording on a reversible thermosensitive recording medium and / or an erasing device for erasing the color recording, the recording medium is heated to a temperature higher than room temperature before the color recording device and / or the recording erasing device. A recording device and a recording / erasing device for a reversible thermosensitive recording medium having a pre-heating device for heating the recording medium are provided.

The reversible thermosensitive recording medium used in the present invention instantly develops color upon heating, and its color-developed state is stable even at room temperature, but the recording layer in the color-developed state is decolored by heating below the color-developing temperature. The decolored state is stable even at room temperature. The principle of coloring and erasing of the reversible thermosensitive recording medium used in the present invention, that is, the principle of image formation and image erasure will be described with reference to the graph shown in FIG. The vertical axis of the graph represents the color density and the horizontal axis represents the temperature. The solid line 1 shows the image forming process by heating and the broken line 3 shows the image erasing process by heating. A is the density in the completely erased state, B is the density in the saturated coloring state when heated to a temperature of T ₁ or higher, C is the density at the temperature of T ₀ or lower in the saturated coloring state, and D is T _0. It shows the concentration when erased by heating at a temperature between T _{1 and} T ₁ .

The reversible thermosensitive recording medium used in the present invention is T
It is in a colorless state (A) at a temperature of ₀ or lower. To perform recording, heating with a thermal head or the like to a temperature of T ₁ or higher is performed, and color development (B) is performed to form a recorded image. Even if the recorded image is returned to the temperature of T ₀ or less according to the solid line 2, the state (C) is maintained as it is, and the recording memory property is not lost. Next, in order to erase the recorded image, the formed recorded image may be heated to a temperature between T _{0 and} T _{1, which} is lower than the coloring temperature, and the state becomes colorless (D). This state is maintained as it is even if the temperature is returned to T ₀ or lower (A). That is, the process of forming the recorded image reaches C by the path of the solid line ABC and the recording is held. During the process of erasing the recorded image, the erased state is maintained up to A by the path of the broken line CDA. The behavioral characteristics of formation and erasure of the recorded image are reversible and can be repeated many times.

FIG. 2 is an explanatory view showing an example of image formation and image erasure. 1 is a support, 2 is a reversible thermosensitive recording layer, and 3 is
Is a colored image. In the image forming process (A) → (B), the image forming heat source, for example, the thermal head 4 is used to perform the T of FIG.
Recording and printing may be performed at a temperature of ₁ or higher. The image erasing step (B) → (A) is achieved by heating the image erasing heat source, for example, the heating roller 5 to a temperature between T _{0 and} T ₁ . The reversible thermosensitive recording medium used in the present invention contains a color former and a developer as essential components. The color development of the reversible thermosensitive recording medium is obtained by cooling the color former composition formed by heating and melting the developer and the color former in the recording layer to room temperature. Since this color-forming material composition has an erasing temperature region on the lower temperature side than the melting temperature, rapid cooling is preferable when cooling from the melt-colored state to room temperature while maintaining the color development.
In the case of gradual cooling, some bleaching occurs when passing through the erasing temperature range, and the density often decreases.

It is considered that in the color former composition, molecules of the color former and the developer interact with each other to open the lactone ring of the color former to develop a color. The composition in the melted state and the rapidly cooled state contains, in addition to the chromophore molecule, the developer molecule and the chromophore molecule which are not directly involved in the formation of the chromophore. In the reversible thermosensitive recording medium used in the present invention, the color former composition at room temperature is in a solidified state due to the cohesive force between these molecules. Further, although the aggregate structure of the color former composition shows some regularity, it may be very regular or not very regular. This depends on the combination of the color developer and the color developer, the amount ratio, and the cooling conditions. Such an aggregate structure
Formation is based on the cohesive force that acts between the alkyl chain structure part of the developer molecule forming the color former and the excess alkyl chain structure part of the developer molecule not forming the color former. Presumed to be done. The formation of such an aggregate structure is associated with the decoloring phenomenon of the color former composition.

The color former composition can be decolored by heating its color development state to a specific temperature range.
In this decoloring process, the aggregate structure of the color developing state changes, and eventually the developer molecules are separated and crystallized from the color former composition to form crystals of the developer alone, resulting in a stable decolored state. X
Confirmed by lines. As described above, in the reversible thermosensitive recording medium used in the present invention, it is clear that the alkyl chain portion of the color developer plays a large role in the formation of the color-developed state and the decoloring process thereof. This is a characteristic of the color forming composition formed on the recording medium. Therefore, the decoloring temperature can be controlled by the length of the alkyl chain portion of the color developing agent, and the longer the chain length, the more often the coloring and decoloring temperatures shift to the high temperature side. This is because the cohesiveness and motility of the developer molecules change depending on the length of this portion.

The reversible thermochromic composition formed in the recording layer of the recording medium used in the present invention is basically a composition in which the color developer having an alkyl chain structure and the color developer are combined. There are preferred color formers for each individual developer. The combination of the color-developing agent and the color-developing agent used in the reversible thermochromic composition is a color-developing state composition obtained by heating both of them to a melting temperature or higher, and decoloring that occurs when the composition is heated to a temperature lower than the melting temperature. It is appropriately selected depending on the ease of decolorization (erasability) and the characteristics such as the color tone of the coloring state. Among these, the decolorizing property is the differential thermal analysis (DTA) of the color-developed state composition obtained by the combination,
Alternatively, it can be determined by the presence or absence of an exothermic peak that appears in the temperature rising process in differential scanning calorimetry (DSC). This exothermic peak corresponds to the decoloring phenomenon that characterizes the composition, and serves as a criterion for selecting a combination having a good decoloring property. In the reversible thermosensitive recording medium used in the present invention, the third substance may be present in the reversible thermosensitive recording layer, and the reversible decoloring behavior can be maintained even if, for example, a polymer compound is present. it can.

In the reversible thermosensitive recording medium used in the present invention, the color developing agent used in combination with the color developing agent has a structure showing a color developing ability capable of developing the color developing agent in the molecule, and an intermolecular structure. It is a compound that also has an alkyl chain structure part that controls cohesive force, such as an organic phosphoric acid compound, an aliphatic carboxylic acid compound or a phenol compound having an aliphatic group having 12 or more carbon atoms, or an aliphatic group having 10 to 18 carbon atoms. It is a metal salt of mercaptoacetic acid or an alkyl ester of caffeic acid having an alkyl group having 5 to 8 carbon atoms.
The aliphatic group includes a linear or branched alkyl group and alkenyl group, and may have a substituent such as halogen, an alkoxy group and an ester group. The reversible thermochromic composition constituting the recording medium used in the present invention is a combination of the color developer and the color developer. The color former is a colorless or light-colored dye precursor exhibiting an electron donating property,
There is no particular limitation, and conventionally known triphenylmethanephthalide-based compounds, fluorane-based compounds, phenothiazine-based compounds, leukoauramine-based compounds, indolinophthalide-based compounds and the like are used. Specific examples of the color developer and the color developer used in the present invention are described in Japanese Patent Application No. 3-355078.
The details are described in Japanese Patent Application Nos. 4-191643 and 4-207604.

In the reversible thermosensitive recording medium used in the present invention, the ratio of the color former to the developer in the thermosensitive recording layer needs to be appropriately selected depending on the physical properties of the compound used. The range is generally in a molar ratio of 1 to 20 for the color developer and 1 to 20 for the color developer, and preferably 2 to 10. If the amount of the developer is less or more than this range, the density of the color-developed state becomes low, which is a practical problem. Further, even in the above preferable range, the decoloring property changes depending on the ratio of the color developing agent and the color developing agent. When the color developing agent is relatively large, the decoloring start temperature is low, and when it is relatively small. Decolorization becomes sharp with respect to temperature. Therefore, this ratio must be appropriately selected according to the application and purpose. The reversible thermosensitive recording medium used in the present invention is one in which a recording layer containing the above-mentioned reversible thermochromic composition is provided on a support, and the basic constitution of the recording medium is the lowermost support. And a recording layer and a protective layer are sequentially laminated thereon. The support used here is paper, synthetic paper, a plastic film or a composite thereof, a glass plate, and the like, as long as it can hold the recording layer.

The recording layer may be in any form as long as the above-mentioned reversible thermochromic composition is present. Usually, the developer and the color former are sufficiently well dispersed in the binder resin to form the recording layer. And a reversible thermosensitive recording medium having a long life can be obtained by this method. The color-developing agent and the color-developing agent can be used as they are or by being encapsulated in microcapsules. Microencapsulation of the color developer and the color developer can be carried out by a known method such as a coacervation method, an interfacial polymerization method or an in situ polymerization method. The developer and the color former may be used alone or in combination of two or more. The recording layer may be formed by a known method in which a color former and a developer are uniformly dispersed or dissolved in water or an organic solvent together with a binder resin, and this is coated on a support and dried. The main role of the binder resin in the recording layer is to prevent the reversible thermochromic composition from aggregating due to repeated coloring and decoloring, and to keep the composition uniformly dispersed. Since the composition often agglomerates when heat is applied during color development, it is preferable to use a binder resin having high heat resistance.

Since the protective layer prevents deformation and discoloration of the surface due to heat and pressure when heat is applied, it is extremely effective to install the protective layer when used many times. In addition, the protective layer may have a role of improving chemical resistance, water resistance, abrasion resistance, head matching property, and the like. Therefore, it is preferable that the protective layer forming material has high heat resistance and high strength, and silicone rubber, silicone resin, polysiloxane graft polymer, ultraviolet curable resin, electron beam curable resin and the like are used. By forming such a protective layer, heat resistance is improved, and resistance to contact with organic solvent, plasticizer, oil, sweat, water, etc. is also increased, and image formation and deletion can be repeated without problems even in a bad environment. A recording medium is obtained. In addition, if a light stabilizer is contained in the protective layer, the light resistance of the image and the background is remarkably improved, and the addition of a polymeric cationic conductive agent prevents electrification, and the addition of an organic or inorganic filler and a lubricant. Thus, the sticking development can be reduced. The protective layer may be formed by uniformly dispersing or dissolving the components of the protective layer in water or an organic solvent as in the case of forming the recording layer, and then uniformly coating and drying this on the recording layer. The thickness is preferably about 0.5 to 10 μm.

The reversible thermosensitive recording medium used in the present invention may be provided with an undercoat layer or an intermediate layer having various purposes. The undercoat layer is provided for the purpose of improving heat insulation, improving the adhesiveness between the support and the recording layer, and improving the resistance of the support to the solvent when forming the recording layer. One of the important roles of the undercoat layer is to improve heat insulation so that the applied heat energy can be used for recording formation and erasure without waste. Coloring and decoloring can be performed sharply by installing the heat insulation undercoat layer. be able to. The undercoat layer for the purpose of heat insulation may be formed by coating fine hollow body particles of an organic or inorganic material on a support, and specifically, a particle diameter of 10 to 10 formed of glass or ceramics, plastic, or the like.
It is sufficient that a minute hollow body of about 50 μm is well dispersed in a solvent together with a binder resin, and uniformly coated and dried on the support.

The method for forming and erasing a recorded image is not particularly limited as long as the temperature conditions for coloring and erasing are given, and a thermal head, laser heating, a hot pen, etc. are used for image formation, and a heating is used for image erasing. Rollers, sheet heating elements, heating lamps and the like are generally used. It is also possible to erase the recorded image with the thermal head set to the erasing temperature and to form the recorded image on the recording medium from which the recording has been erased with another thermal head set to the recording temperature.

The essence of the present invention is to preheat the recording medium immediately before applying heat to the reversible thermosensitive recording medium with a thermal head or the like, and the preheating heats the recording medium to an unnecessarily high temperature. Since it is possible to reach the predetermined temperature in a short period of time, it is possible to achieve both high resolution recording image acquisition and long life of the recording medium. It should be noted that the higher the temperature of the recording medium when heat is applied, the higher the temperature of the recording medium can be raised to a predetermined temperature, and an image with good resolution can be obtained. Is preferably raised to the highest possible temperature, and the optimum preheating temperature during image formation is the decoloring temperature of the recording medium.

A typical example of the heating means at the time of preheating performed in the present invention is as follows. Contact the recording medium with a constant temperature heat source such as a heating roller or hot plate. Irradiate electromagnetic waves or sound waves such as light or microwaves onto the recording medium. Heat fluid such as air or water to contact the recording medium. Since these heating means for applying heat with a thermal head have advantages and disadvantages, in consideration of the purpose of use of the recording medium, the heating means at the time of forming an image, the heating means at the time of erasing, etc., it is appropriately selected from the above However, the method is simple in operation, and the device cost of the preheating means is low. In addition, the method (1) has high energy efficiency and enables downsizing of the recording apparatus.

An example of an image forming method incorporating the preheating method of the present invention is shown in FIG. In FIG. 3, the recording medium 6 is first preheated by passing between the heating roller 5 and the heating roller pressing roller 7, and then imagewisely applied by passing between the thermal head 4 and the platen roller 8. Since the preheating temperature in this case is optimally the decoloring temperature as described above, a heating roller for recording and erasing may be used as the heating roller in FIG. 3, and this method separates image formation and image erasing. It is a preferable method when it is carried out. The preheating method of the present invention is particularly important during image formation because it is intended to prevent a partial increase in temperature of the recording medium that occurs when heat is applied by a thermal head, and when the image is erased by a mild heating means such as a heating roller. The effect is smaller than when forming an image. However, when the image is erased by the thermal head, the effect similar to that at the time of image formation can be expected as a matter of course.

[0023]

EXAMPLES Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. All parts shown below are based on weight. The color density and the color density of the recorded image were measured with a Macbeth reflection densitometer.

Example 1 The following composition was pulverized and dispersed by a ball mill to have a particle size of 1 to 4 μm to prepare a coating liquid for forming a recording layer. 2- (o-chloranilino) -6-dibutylaminofluorane 10 parts Stearylphosphonic acid 30 parts Vinyl chloride-vinyl acetate copolymer (manufactured by Union Carbide Co., VYHH) 30 parts Methyl ethyl ketone 135 parts Toluene 135 parts As described above. The coating solution prepared as described above was applied to a polyester film having a thickness of 100 μm with a wire bar so that the coating thickness was 5 μm, and dried to form a recording layer.

Next, the following composition was applied onto this recording layer, dried, and then irradiated with ultraviolet rays to be cured to form a protective layer having a film thickness of 4 μm. 75% butyl acetate solution of urethane acrylate-based UV curable resin (Unidick C7-157, manufactured by Dainippon Ink and Chemicals, Inc.) 100 parts Butyl acetate 50 parts The reversible thermosensitive recording medium prepared as described above has a coloring temperature of about 120. The decolorization temperature was about 70 ° C. Using this heat-sensitive recording medium, printing was performed by the recording apparatus of FIG. 3 and then the printing was erased by another heating roller. After the temperature of the recording medium returned to room temperature, printing and erasing of printing were repeated 100 times, and the durability of the recording medium was examined. The results are shown in Table 1. The surface temperature of the preheating roller of the recording apparatus was set to 60 ° C, and the surface temperature of the printing erasing heating roller was set to 70 ° C.

Example 2 Using the same reversible heat-sensitive recording medium and recording apparatus as in Example 1, the same repeated test as in Example 1 was conducted with the surface temperature of the heating roller for preheating at 70 ° C., and the results are shown. The results are shown in Table 1. Since the image is erased during the preheating in this example, the image erasing step and the preheating step are combined in the repeated test. Comparative Example 1 The same test as in Example 1 was performed except that preheating was not performed, and the results are shown in Table 1.

Example 3 A recording apparatus having two thermal heads as shown in FIG. 4 was manufactured. In this recording apparatus, the first thermal head performs preheating, and the second thermal head performs image formation. That is, the recording medium 6 is the first thermal head 4
And the platen roller 8 and is preheated,
Subsequently, heat is applied by the second thermal head to form an image. The same reversible thermosensitive recording medium as in Example 1 was fed into the recording apparatus, and the same repeated image forming and image erasing tests as in Example 1 were tried. In this case, the image erasing and preheating were simultaneously performed by the first thermal head, so the heating roller for image erasing was not used. The test results of this example are shown in Table 1.

Comparative Example 2 The distance between the two thermal heads was increased so that after the image was erased by the first thermal head, the recording medium was cooled to room temperature and then fed to the second thermal head. . Otherwise, the same image forming and image erasing tests as in Example 3 were repeated, and the results are shown in Table 1.

[0029]

[Table 1]

From Table 1, in the experiment of the comparative example without preheating, the color density after 100 times of repetition was lower than that of the experiment in the example, and the decolorization density was high. This shows that the recording medium used in the experiment of the comparative example is deteriorated from the recording medium used in the experiment of the example, and the effect of preheating is clear. Further, it can be seen from Table 1 that the heating roller is superior to the thermal head as a heating means for image erasing and preheating.

[0031]

According to the present invention, the recording medium is preheated before the formation and / or erasing of the image, so that the heating of the recording medium can be prevented in a short time and at an unnecessarily high temperature. As a result, thermal deterioration of the recording medium can be prevented and a clear color image can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the color density and the temperature of a reversible thermochromic composition used in the present invention, and is an explanatory view of the principle of color development and decolorization. The solid line (A → B → C) shows the color developing process, and the broken line (C → D → A) shows the color erasing process.

FIG. 2 is an explanatory diagram of an image forming process and an image erasing process of the reversible thermosensitive recording medium used in the present invention.

FIG. 3 is an explanatory view showing an example of an image forming step in the case of including a preliminary heating step of the present invention, in which a heating roller is used as a heating means for preliminary heating.

FIG. 4 is an explanatory view showing another example of an image forming step in the case of including a preliminary heating step of the present invention, in which a thermal head is used as a heating means for preliminary heating.

[Explanation of symbols]

 1 Support 2 Reversible Thermosensitive Recording Layer 3 Colored Image 4 Thermal Head 5 Heating Roller 6 Reversible Thermosensitive Recording Medium 7 Heating Roller Pressing Roller 8 Platen Roller

Claims (3)

[Claims] 1. A color-erasing state in which a recording layer containing an electron-donating color-forming compound and an electron-accepting compound is provided, and a color-recorded state is formed by heating and melting, and the recording disappears when heated below the color-recording temperature. Forming a reversible thermosensitive recording medium on which color recording is performed, and / or an erasing step of erasing the color recording, the recording medium is cooled to room temperature immediately before the color recording step and / or the recording erasing step. A recording method and a recording erasing method for a reversible thermosensitive recording medium, characterized in that a preheating step of heating to a higher temperature is additionally provided. 2. The method according to claim 1, wherein the preheating performed immediately before the color recording step is performed at a recording / erasing temperature of the reversible thermosensitive recording medium. 3. A color-erased state in which a recording layer containing an electron-donating color-forming compound and an electron-accepting compound is provided, a color-recorded state is formed by heating and melting, and the recording disappears when heated below the color-recording temperature. In a device including a color recording device for color-recording a reversible thermosensitive recording medium that forms a recording medium and / or an erasing device for erasing the color recording, the recording medium is placed at room temperature in front of the color recording device and / or the recording / erasing device. A recording device and a recording erasing device for a reversible thermosensitive recording medium, which is equipped with a preheating device for heating to a higher temperature.